Recently, as the transmission traffic increases, there are increasing needs for introducing optical transmission systems of the next generation which have transmission capacities of 40 Gbit/s or greater. Various modulation schemes having higher spectral efficiencies, optical signal-to-noise ratio (OSNR) tolerance, and non-linearity tolerance as compared to the Non Return to Zero (NRZ) modulation scheme that has been employed in conventional system are viewed as promising for realizing such systems.
Among them, multi-level phase modulation schemes, such as (differential) quadrature phase-shift keying ((D)QPSK) modulation scheme, are regarded as promising as modulation schemes for optical transmission systems of the next generation. This is because multi-level phase modulation schemes have properties, such as higher dispersion tolerance, higher polarization mode dispersion (PMD) tolerance, and narrower spectrum. As a technique to realize further improvements in the characteristics (OSNR tolerance, chromatic dispersion tolerance) of such multi-level phase modulation schemes, a digital coherent receiving scheme that combines the coherent reception with digital signal processing has been proposed (for example, Patent Reference 1, Non-Patent Reference 1, or the like).
(Patent Reference 1) U.S. Pat. No. 7,315,575
(Non-Patent Reference 1) S. Tsukamoto, et al., “Optical Homodyne Receiver Comprising Phase and Polarization Diversities with Digital Signal Processing,” Mo4.2.1, European Conference on Optical Communication 2006, 2006”
It is desired that the signal quality of data recovered at the receiver side of a multi-level phase modulation communication system having a transmission capacity of 40 Gbit/s or higher (baud rate of 20 Gbit/s or more) is further enhanced.
Accordingly, one object of the present application is to enhance the signal quality of data recovered on the side that receives multi-level phase modulated signal lights.
Note that, other than the above-identified object, it can be regarded as an object of the present invention to provide advantages and effects that can be obtained by the best modes to implement the invention described below but cannot be obtained with conventional techniques.